A variety of medical instruments have been developed for use in monitoring and treating patients. Many of these instruments are designed to be electrically coupled to the patient via one or more electrodes. The electrodes receive electrical signals from, or transmit electrical energy to, some portion of the patient's body.
In that regard, a defibrillator/monitor typically includes two or more monitoring electrodes that receive electrical signals from the patient's heart. These signals are then commonly displayed by the monitor, allowing the attending physician to evaluate the heart's operation. In addition, a pair of defibrillation electrodes are used to transmit electrical energy from the defibrillator to the patient to, for example, terminate undesired fibrillation of the heart.
The monitoring and defibrillation electrodes used with the defibrillator/monitor are often applied externally to the patient's chest and/or limbs. As will be appreciated, the impedance of the electrodes, the transthoracic impedance of the patient, and the impedance of the electrode/patient interfaces, all influence the signals received by the monitor and the energy delivered to the patient. Typically, the electrodes are designed to reduce the influence of impedance on the instrument's operation as much as possible.
In that regard, external electrodes are made relatively large to reduce the impedance of the electrode/patient interface. Also, a conductive gel is often applied to the surface of each electrode before the electrode is attached to the patient to further limit the interface impedance. Despite such precautions, the impedance of the electrode/patient interface may still have undesired influences on the instrument's operation.
One of the most common problems involving electrode/patient impedance is related to motion. For example, with monitoring electrodes applied to a patient's chest, movement of the patient or the electrodes may disturb the patient/electrode interface. The resultant variations in interface impedance introduce corresponding variations in the electrical signals received at the monitor, independent of the operation of the heart. This "motion artifact" in the monitored signal can, in turn, cause the instrument or operator to erroneously interpret the condition of the heart.
Relative motion between the patient and defibrillation electrodes may similarly be of interest. For example, patient motion may indicate that the patient is conscious or is being moved by a health care provider. In either instance, it may be undesirable to discharge energy to the patient. Further, motion-induced variations in the impedance of the electrode/patient interface may result in corresponding variations in energy losses at the interface. Thus, the energy actually delivered to the patient to terminate fibrillation may differ considerably from that selected by the operator.
Prior art systems have been developed to address these limitations. In that regard, some systems monitor the impedance at the electrode/patient interface to determine when motion is occurring. In the event the monitored impedance suggests that motion is occurring, operation of the instrument is then inhibited.
By way of illustration, U.S. Pat. No. 4,919,145 (Marriott), assigned to Physio-Control, reviews a number of different techniques used to sense lead impedance and/or transthoracic impedance (TTI). In that regard, the background section of the Marriott patent indicates that a small DC signal can be applied to the leads, with the resulting DC voltage across the leads then being representative of impedance. Another approach described in the background section of the Marriott patent involves the application of a high-frequency, constant current signal to the leads. The Marriott patent then goes on to disclose an arrangement in which two carrier signals are used to detect a lead impedance related voltage and an impedance respiration related voltage.
U.S. Pat. No. 4,619,265 (Morgan et al.), also assigned to Physio-Control, discloses an arrangement in which a patient's TTI is evaluated to detect motion. More particularly, TTI signals are compared against some predetermined threshold level. If the last two measurements of TTI exceed the threshold, a display is generated prompting the operator to stop all motion. If motion is detected for more than fifteen seconds, the operator is also prompted to perform cardiopulmonary resuscitation.
With only one or two impedance measurements used to detect motion, temporary aberrations in the measurements due, for example, to noise are likely to influence the detection of motion. In that regard, noise in the measured impedance signal may cause the signal to be erroneously high or low at any given time. Although the resultant signal variations may average out over time, with only one or two measurements used, the measurements are likely to be inaccurate. As will be appreciated, it would be desirable to allow motion to be detected in a manner that is relatively free from the influence of noise.
As disclosed by Morgan et al., the use of limits detection plays an important role in conventional motion detection schemes, allowing an impedance measurement to be compared against some predetermined threshold level associated with motion. In accordance with the present invention, limits detection plays roles both in the processing of impedance data used in the detection of motion and in the processing of monitored cardiac signals used to evaluate the condition of the patient's heart.
In that regard, the signals used to monitor cardiac activity and electrode impedance are conventionally filtered by a preprocessing circuit prior to analysis. Filtering is performed to remove select portions of the signals, preserving only those portions that have a high information content. The removed portions may be attributable to, for example, some baseline signal contributor or noise.
The conventional filter circuits used often employ capacitive elements, as well as resistive and inductive elements. When the signal applied to such a filter circuit undergoes large deviations, the capacitors may become fully charged, rendering the filter inoperative until the charge stored on the capacitors has time to decay. As will be appreciated, it would be desirable to determine when the input to such a filter undergoes a large deviation, so that some form of corrective action can be taken to limit the inoperability of the filter circuit.
In view of the preceding comments, it would be desirable to develop a method of detecting limits associated with electrode/patient motion, free from the disruptive influence of, for example, noise. It would further be desirable to develop a method of detecting limits associated with the inoperability of filter circuits conventionally used in medical instruments. To reduce the complexity of the overall processing performed by the instrument, it would further be desirable for the same general method to be used in detecting both types of limits.